1. Field of the Invention
The present invention relates to dental x-ray diagnostic installations and in particular to such installations having a means for controlling the radiation dose rate for obtaining an optimum film exposure.
2. Description of the Prior Art
An x-ray diagnostic installation is disclosed in German Pat. No. OS 24 47 075 which includes an x-ray tube and a cassette holder spaced therefrom which are rotatable about vertical axes around a patient's head for generating a dental x-ray of the patient's jaw and teeth. The cassette holder has a radiation detector attached thereto which generates an electrical signal corresponding to the radiation dose rate when struck by the x-rays. This electrical signal is supplied to a dose rate regulator which controls the x-ray tube voltage such that the dose rate is maintained at a level for producing an optimum film darkening or exposure.
Alternatively, the x-ray tube current may be controlled to achieve the same result. This conventional device operates on the principle that the x-ray tube voltage or current will be altered in accordance with the density characteristics of the patient in such a manner that the mean radiation dose reaching the individual film sections is maintained constant. The density characteristics of a patient may fluctuate greatly during the course of a sweep of the x-ray tube and cassette holder in the course of generating a total fluoroscopy. Additionally, relatively large differences in density characteristics are also observed between individual patients, for example between children and adults. Accordingly, the installation disclosed in German Pat. No. OS 24 47 075 requires a relatively large range of adjustment in order to maintain the radiation dose at a constant level.